Spectrometer with self-referenced calibration

1. An absorption spectrometer comprising: a light detector and a light source providing a beam directed along a path; a sample holder providing a chamber adapted to hold a sample in the path through which the beam may pass, the sample holder movable to change the length of the path through the sample between a first and second length; an electronic processor communicating with the light detector to make measurements of light intensity of light along the path at the first and second length to provide an output indicating absorption of light by the sample isolated from variations in light from influences outside of the sample; and wherein the chamber provides an entrance window accepting the beam from the light source into the sample and an exit window discharging the beam from the sample to the light detector, wherein the sample holder moves the entrance window with respect to the exit window to change a distance therebetween.

2. The absorption spectrometer of claim 1 wherein the chamber changes its dimension along the path to change the path length.

3. The absorption spectrometer of claim 1 wherein walls of the chamber deform to change the path length through the sample.

4. The absorption spectrometer of claim 1 wherein the path is a straight line between the light source and light detector.

5. The absorption spectrometer of claim 1 wherein the beam at the first length and second length follows a common path between the light detector and light source.

6. The absorption spectrometer of claim 1 wherein the beam has a polarization that is unchanged between measurements of the beam along the first length and second length.

7. The absorption spectrometer of claim 1 wherein the electronic processor combines the measurements along the first length and second length by dividing one measurement by the other.

8. The absorption spectrometer of claim 1 wherein the first and second lengths stand in a ratio of at least 1:1.5.

9. The absorption spectrometer of claim 1 wherein the chamber is a conduit adapted to allow for a continuous unidirectional flow of liquid therethrough.

10. The absorption spectrometer of claim 9 further including a liquid sample.

11. The absorption spectrometer of claim 10 wherein the liquid sample is a liquid fuel.

12. An absorption spectrometer comprising: a light detector and a light source providing a beam directed along a path; a sample holder providing a chamber adapted to hold a sample in the path through which the beam may pass, the sample holder movable to change the length of the path through the sample between a first and second length; an electronic processor communicating with the light detector to make measurements of light intensity of light along the path at the first and second length to provide an output indicating absorption of light by the sample isolated from variations in light from influences outside of the sample; wherein the chamber is a conduit adapted to allow for a continuous unidirectional flow of liquid therethrough; further including a liquid sample wherein the liquid sample is a liquid fuel; and further including an engine controller for adjusting at least one of timing and igniter power according to the output indicating absorption of light by the sample.

13. The absorption spectrometer of claim 1 wherein the absorption spectrometer is an FTIR spectroscope and wherein the electronic computer applies a Fourier transform to a received signal to extract a spectrum.

14. The absorption spectrometer of claim 1 wherein a changing in separation between the entrance window and exit window define the first and second paths.